Chemical heating pads of the type activated by exposure of a chemical mixture to atmospheric air reaching the mixture through openings in an envelope which contains the mixture are well known and have been known for many years.
Numerous patents have been granted for improvements in such chemical heating pads over the course of many decades, including the following U.S. patents: U.S. Pat. Nos. 1,434,576 (Wertheimer); 1,609,958 (Perrault); 1,620,581 (Smith); 3,301,250 (Glasser); 3,976,049 (Yamashita et al.); 4,106,478 (Higashijima); 4,282,005 (Sato et al.); 4,366,804 (Abe); 4,516,564 (Koiso et al.); and 4,573,447 (Thrash et al.).
A typical example is U.S. Pat. No. 3,976,049, which discloses a warming pad having an exothermic composition including iron powder, a chloride or sulfate salt, carbon powder, and water in a two-layered bag made of an air-permeable cloth layer inside a film layer with aeration holes. The entire pad is contained in an impermeable envelope which is opened to allow removal of the pad. This allows atmospheric air to pass through the aeration holes and permeable layer of the bag, which exposes the composition to air (specifically its oxygen) to begin the exothermic reaction. This device is typical of such heating pads.
While there have been many developments in the field of chemical heating pads, there remain significant problems and shortcomings with heating pads of this general type. For example, such pads are often too hot or not hot enough for the intended purpose.
It is known that the amount of heat produced, the rate of heat generation, and achievable temperatures are dependent on, inter alia, the chemical composition of the mixture inside the pad, the size and number of holes in the pad exposed to the atmosphere, and the thickness of the material of the pad. These factors often have been considered in the prior art in seeking to provide chemical heating pads meeting particular requirements.
One object of some developments of the prior art has been to achieve constant and predictable heat conditions on the major surfaces of an air-dependent chemical heating pad. In certain other cases the object has been to have heat emitted from only one side of such a heating pad, with the second side shielded or insulated to prevent heat emission. These objects contrast sharply with a principal object of this invention, that is, to achieve different heat-generation levels and different useful heat conditions when opposite sides of a pad are applied to the body or other surface to be heated.
When chemical heating pads are used on human skin for various purposes, they are frequently either too hot or not hot enough for the skin of the users. That sensitivities to topically-applied heat vary greatly in this way is established. It is also known, in the field of therapeutic heat treatment, that heat sensitivity varies not only from person to person but according to the location on the body of the person being treated. For any person, some areas of skin are better able to receive applications of heat.
The inability of prior air-dependent chemical heating pads to adequately satisfy varying sensitivities of users is well known, and has limited the use of such heating pads. Pads with only a single level of heat generation and heat transfer or only one heat-transfer surface are simply not suitable for therapeutic use unless special accommodations are made. There is a long-standing and clear need for an improved self-contained heating pad.